Having a lead character be perfect can actually be quite boring. You can always predict exactly what they will do or how a situation involving them will play out. Since they can't make mistakes there is no tension. This goes for villains too. Flaws make lead characters more interesting. Since we aren't perfect either, we can relate to them, especially if they struggle with things we would also struggle with. Often the hero only succeeds due to blind luck or chance which allows for constant tension, since their luck may run out at any moment.

Having a faithful servant/ sidekick who is perfect and lacks flaws gives us a contrast to the hero and throws their flaws into sharp relief. They also provide a way to get the lead character/hero out of trouble OR if the sidekick is incapacitated, or killed, they can create tension by forcing the hero to overcome their problems on their own.

This creates an interesting scenario where you may find you prefer the sidekick to the hero, but importantly this is only because they are constantly saving the hero, often showing selfless devotion. The key point here is without the hero the sidekick would have no one to play off against, no one to save and no way to show their selflessness. It's a symbiotic relationship.

For the quintessential example of this, see Jeeves, from P.G. Wodehouse's Jeeves & Wooster series.

Finally, can you think of any examples of when you didn't like a sidekick as much when they were made into the main character? It's most common in movies where the sidekick might get a spin off, or more screen time in sequels. Captain Jack Sparrow in the later Pirates of the Caribbean movies is a great example of this.